Record from Telephone Headset Connector

The other day someone called with an interesting problem. He wanted to
record the far side of a telephone conversation using his laptop sound device,
and he did not have access to a regular telephone line to use one of the
telephone recording interfaces described here. He was using a VOIP
phone device that plugs
into a USB port, so no regular POTS line
is involved. The device provides a connector for a standard telephone headset,
which combines microphone and earphones. In addition, he wanted to still be
able to use his telephone headset while recording, and keep everything
low-cost, avoiding soldering if possible.

In this article we describe a few simple do-it-yourself recording solutions
for headsets. This is also applicable for recording from a cell phone or
cordless phone with headset connector, as well as the VOIP phone described
above. These solutions might NOT work with land-line phones having headset
connectors, because of possible ground
loop problems. We will cover those phones in a later tutorial. When
possible, we give Radio Shack part
numbers-- where omnipresence counts more than quality.

Technical Background

Telephone headsets most often use 2.5mm (3/32") male connectors, smaller
than the 3.5mm (1/8") audio connectors found on your computer. Here is the
pin-out of the headset plug:

Compare this to the pin-out of your line-input connector:

and microphone connector:

Voltage levels for Line-In are on the order of 1Volt, and the voltage level
of a microphone is on the order of 10-100mV.

Experimenting with two different cell phones and two models of cordless
phone, we find the speaker-phone is disabled when the headset plug is inserted,
preventing us from monitoring the phone call while the headset connector is
plugged, using the same phone.

Sidetone refers to the
fact that audio that we speak in the telephone microphone is added to what we
hear at the telephone earpiece. This is used as feedback so we don't talk to
loud and distort audio, but more importantly for this application, this allows
us to record the audio at the earpiece and get both sides of the conversation.

Record from Cordless Headset Connector, Monitoring With Headset

In addition to a wired telephone headset and stereo audio patch cable with
3.5mm plugs at each end, we need two parts. RS 274-373
[1/8" to 3/32" Adapter]

and connect the other end of the audio patch cable into computer sound
device "Line-In" connector. Set up your audio recording software to
record in Mono, and 8000Hz sample rate is fine. Here is a sample recording
using this set-up:

Record from Cordless Headset Connector, Monitoring With Extra Extension

Can we record with even fewer parts? The answer is yes! Yes, assuming we
have another extension. Remember, if we plug anything into the headset
connector, we cannot use that telephone the normal (non-headset) way. Well,
suppose we have another phone extension, so one phone is used for talking and
listening, and the other is used for recording only.

Please remember to take both extensions off-hook at the beginning of the
call. Also note that some cordless phones have a "privacy mode" feature: if any
extension is in use, no other extension can listen. For this recording trick,
you will need to turn off privacy mode for your phones.

Record from POTS Wired Headset Plug

We will address this in a following tutorial. (Actually, this might be
tricky, since we no longer have land-line to the office. We'll have to find a
nearby volunteer.)

Summary

We presented simple solutions for recording from telephone headset connector
to computer sound device. For other telephone recording solutions, such as
interfacing to the telephone line or handset connector, see our other technical resources.

Commercial Products

We haven't tried any of these! Note that some recording interfaces assume a
battery-operated tape recorder, so they leave out circuitry to prevent
ground-loop noise to reduce expense.